The present invention relates to cured unsaturated polyesters as typified by cured sheet molding compound (SMC) and more particularly to the treatment of cured unsaturated polyester for recovery of valuable recycle products.
Thermosets, e.g. phenolics, epoxies, unsaturated polyesters, are widely used as binders in reinforced plastic and composites, where the thermoset can comprise up to 25%-80% of the total composite, and fillers, such as glass (roving, mat, fiber), calcium carbonate and silica comprise the balance. These thermoset binders with specific fillers produce composites of high mechanical properties, chemical resistance, and high thermal stability. A more recent development is sheet molding compound (SMC).
In the United States, The Society of Plastics Industry (SPI) introduced the term SMC to differentiate this material from pre-pregs. One outstanding advantage is that SMC can be molded in one step at a rate approaching the steel-stamping operations of the automotive industry. This characteristic, coupled with high mechanical strength (impact in particular), corrosion resistance, and metallic appearance when painted, has made SMCs popular for the production of automotive and truck hoods, tops, fenders, doors, and in fact, virtually the entire body.
Within the next 5-10 years, given the relatively short life of automobiles, SMC parts will accumulate as they cannot be scrapped in a manner similar to those made from metals. It is, therefore, critical to establish the basic technology required for the economic recycling of SMC parts.
Cured SMC and the like, being thermoset materials, have had the reputation for being non-recyclable because the thermosetting process used in their manufacture is irreversible. Reinforced Plastics, p. 46 (Oct. 1990). Thermosets also cannot be remolded into new products after use. "Taking the Heat Out of Thermoset", Materials Edge, No. 33, p. 9 (Feb. 1992). Lantos, "Are Plastics Really the Landfill Problem?", Chem Tech., Aug. 1990, 473, excludes thermosets when discussing the recycling of plastics since thermosets are non-meltable.
Those researchers that have confronted the accepted notion that thermosets are non-recyclable generally prefer attrition and separation of the thermoset resin from the filler, with reuse of all fractions as reinforcement. Such is the basis for the ERCOM program in Germany. "Recycling in Action", Reinforced Plastics, Feb. 1992, 32-35. Jutte, et at., "Recycling SMC", 46th Annual Conf., Composites Institute, The Society of Plastics Industry, Inc., Feb. 18-21, 1991, reports a variety of approaches to recycling of thermosets including incineration, chemical degradation, pyrolysis, and size reduction.
The chemical degradation of thermosets typically has focused on hydrolysis reactions. For example, Tesoro, et al., report the use of laboratory-produced, untilled, ideal unsaturated polyester systems which are subjected to neutral hydrolysis with equivocal results being reported. "Chemical Products from Cured Unsaturated Polyesters", Proceedings of the American Chemical Society, Division of Polymeric Material Science and Engineering, Vol. 67, 459-460 (1992); "New Concepts for Recovery of Thermosets", 47th Annual Conf., Composites Institute, The Society of Plastics Industry, Inc., Feb. 3-6, 1992; "New Concepts for Recovery of Thermosets". Patel, et al., "Recycling of Sheet Molding Compounds (SMC)", Proceedings of the American Chemical Society, Division of Polymeric Material Science and Engineering, Vol. 67,455-456 (1992), report that hydrolysis of extracted ground SMC under vigorous hydrolytic conditions even for long periods gave poor yields of about 1.5%. Patel, et al., "Alternative Procedures for the Recycling of Sheet Molding Compounds", Advances in Polymer Technology, Vol. 12, No. 1, 35-45 (1993), also reports hydrolysis yields to be about 1.5 %. It should be noted that Kinstle, et al., "Chemical Intermediates from Scrap Polymers Via Hydrolysis", Polymer Preprints, Vol. 24, No. 2, 446-447 (Aug. 1983) reports the neutral hydrolysis of styrene-cross linked unsaturated polyesters at 200.degree.-275.degree. C. for 2-12 hours with reported products being isophthalic acid, styrene-fumaric acid copolymer, and unhydrolyzed polyester feedstock.
Clearly, the art has identified a compelling need to enable manufacturers to recycle cured unsaturated polyesters, and hopefully make such recycle economic and efficient.